1. Field of the Invention
The present invention relates to the field of aeronautics, and more particularly, a process for measuring the deformation of an aircraft blade during its operation.
2. Description of the Related Art
Conventionally, an aircraft turbo-machine includes rotors comprising a plurality of radial blades to accelerate an air flow from upstream to downstream in the body of the turbo-machine. Performances of a blade mainly depend on the blade shape when the same is rotationally driven with the rotor it is mounted to. In reference to FIG. 1A, a blade 1 is mounted on a turbo-machine shaft 2 extending along an axis U. The blade 1 has traditionally a tridimensional shape which is changed as a function of the rotation speed of the turbo-machine shaft 2. By way of example, in reference to FIG. 1B, the blade 1 can elongate radially and/or twist when the rotation speed of the turbo-machine shaft 2 increases.
To determine an optimum shape for the turbo-machine blade, a theoretical model of the blade is conventionally used, which mathematically defines the blade shape, for example, by means of a meshing. In a known manner, such a mathematical model further enables stresses globally applied to the blade to be defined when the same is locally deformed. In other words, if the blade head elongates by 3 mm, the theoretical model allows to predict what the blade global deflected curve is, from its head up to its shank, as well as stresses applied to the blade.
A theoretical model of a blade is, by nature, defined from mathematical hypotheses that need to be practically validated to certify the conformity of the theoretical model. To do this, the blade deformation is measured during the operation of the turbo-machine and the measurement of the deformation is compared to a theoretical measurement provided by the theoretical model. In case of a deviation, the mathematical parameters of the theoretical model are modified for the theoretical deformation measurement to correspond to the “actual” deformation measurement measured during tests. This step is conventionally designated by those skilled in the art as a step of “resetting the theoretical model”.
Thus, the conformity of a theoretical model directly depends on the measurement accuracy of the blade actual deformation during operation of the turbo-machine.
In reference to FIG. 2, for a turbo-machine including an axial turbo-machine shaft 2, to which radial blades 1 are mounted, and a fan case 21 wrapping the blades 1, position sensors C are conventionally placed onto the inner surface of the case 21 so as to detect the passage of the heads 10 of the blades 1 on the same level as the sensors C. By way of example, the position sensors C are in the form of optical sensors which enable the shutting time of the optical beam to be measured during the passage of a blade head. This shutting time directly depends on the blade head shape during its rotation which enables the fundamental deformation mode of the blade, for example a blade twisting, to be conventionally inferred. This deformation measurement method is known to those skilled in the art as “tip timing”.
When the turbo-machine does not include an outer case, which is for example the case of so-called “open rotor” contra-rotating propeller turbo-machines, it is not possible to implement the “tip timing” method since the turbo-machine does not include a support for position sensors. A solution to suppress this drawback would be to provide a support strut for the position sensors so as to position said sensors external to said propellers in order to simulate a deformation measurement by “tip timing”. In practice, this solution is to be avoided because the support strut induces a change in the aerodynamic behaviour of the turbo-machine propellers and thus a worsened accuracy in the deformation measurement.
A deformation measurement by “tip timing” also has this drawback that it only enables a local deformation at the end of the blade to be detected. It is then difficult to infer the global deformation of the blade therefrom, in particular, when the blade has preferential stiffness directions which is the case, for example, for a blade of composite material.